EFFECTS OF INHALED ANESTHETICS ON MUCOCILIARY FUNCTION
Normal Mucociliary Function
Foreign particulate matter, microorganisms, and dead cells are
removed by the upward clearance of mucus from the tracheobronchial tree as a primary
pulmonary defense mechanism. Ciliated respiratory epithelium extends throughout
the respiratory tract as far as the terminal bronchioles and decreases in density
(as do mucus-producing goblet cells and submucous glands) from trachea to alveoli.
[90]
Ciliary motion consists of a rapid stroke
in
a cephalad direction, followed by a slower recovery stroke in the opposite direction.
Movements of cilia are closely coordinated in a proximal-to-distal direction to
move matter toward the trachea efficiently. This resulting wave of motion is known
as metachronism. The bending of individual cilia
results from ATP-dependent sliding of two parallel fibers within the ciliary filament,
but it does not appear to involve the autonomic nervous system.
Mucus represents a mixture of water, electrolytes, and macromolecules
(e.g., lipids, mucins, enzymes) secreted by goblet cells and mucosal glands. The
rheologic properties of mucus influence the rate and efficiency of ciliary function.
Thicker layers of mucus slow the removal of
surface particles from the airway, whereas low-viscosity mucus promotes the most
rapid ciliary transport. The amount and physical properties of the mucous layer
may also promote the coordination of ciliary beats.[91]
Reid[92]
studied the physical and chemical characteristics
of mucus in expectorated specimens. However, the volumetric, rheologic, biochemical,
and clinical examination of normal respiratory secretions is made somewhat difficult
by the contamination by salivary secretions and desiccation of these secretions in
sputum.[93]
Relatively newer methodologies permit
more precise examination of in situ specimens, and some useful information has been
garnered from tracheotomized subjects.
Mucociliary function in single cilia or respiratory epithelial
tissue cultures may be assessed using high-speed videomicroscopy to examine the ciliary
beat frequency. In vivo techniques in experimental animals have employed a tracheal
window model. The velocity of mucus movement has been measured with radioactive
markers or fiberoptic bronchoscopy in humans. The deposition of inhaled radiopaque
or radioactive particles throughout the lung fields, followed by radiographic examinations
of clearance of these particles, allows examination of mucociliary function in peripheral
and central airways, even in small mammals.[94]
Impaired mucociliary function in the upper airways correlates
with low levels of nasal NO, but the clinical significance of this finding remains
to be defined.[95]
The maintenance of bronchial
perfusion is also critically important to the maintenance of normal mucociliary function.
[96]
Although nervous control of ciliary coordination
has not been demonstrated in vertebrates,[93]
mucociliary
clearance, most likely related to changes in physical characteristics of respiratory
secretions, is closely related to autonomic nervous system activity.[91]
